In recent years, protection of pedestrians during car accidents has been regulated and a pedestrian protection performance is attracting attention as an index for rating automobile hoods. Meanwhile, a space under the hood necessary for protecting pedestrians has been reduced due to an engine enlarged in an endeavor of obtaining a high engine output and to a number of parts within an engine room increased for a purpose of multiplying functions. Due to that, it has become important to develop an automobile hood that can absorb impact energy efficiently with a small space to attain both a sporty design and the pedestrian protection performance.
As the automobile hood as described above, JPA2000-168622 (Claim 1, FIGS. 1 through 6) describes an automobile hood having a cross-sectional hood structure interposing airspaces between an outer panel and an inner panel when the both panels are joined. JPA2000-168622 also describes the hood structure in which a plurality of dimples having different depths is formed on the inner panel to create the airspaces described above.
FIGS. 13, 16 and 17 show exemplary automobile hoods having hood structures similar to the hood structure described in JPA2000-168622.
As shown in FIGS. 13(a) through 13(c), the automobile hood (beam type hood structure) 21A is constructed by an outer panel 22 having a predetermined curvature and an inner panel 23 having a concave portion 25 at a peripheral part thereof, a plurality of beams 26 extending so as to appropriately intersect or to run substantially in parallel with each other in vertical, horizontal and oblique directions in a panel plane direction at a center part thereof and voids 26A trimmed among the beams 26. Thus, the automobile hood 21A has a cross-sectional structure in which airspace portions 24 are interposed when the peripheral part of the outer panel 22 is joined with an end of the concave portion 25 of the inner panel 23 by means of a hemming process. Further, bonding portions 27 are disposed at predetermined intervals on tops of side walls of the beams 26 to bond the beams 26 with a back face of the outer panel 22 through the intermediary of the bonding portions 27. Still more, a lock reinforcement 30 and hinge reinforcements 31 are joined to the concave portion 25 as reinforcing members of the automobile hood 21A.
As shown in FIGS. 16(a) through 16(c), an automobile hood (cone-type hood structure) 21B is constructed by an outer panel 22 having a predetermined curvature and an inner panel 23 having a concave portion 25 at a peripheral part thereof and a plurality of cone-like convex portions 28 regularly disposed at a center part of the automobile hood 21B. The automobile hood 21B has a cross-sectional structure in which airspace portions 24 are interposed when the peripheral part of the outer panel 22 is joined with an end of the concave portion 25 of the inner panel 23 by means of a hemming process. Further, bonding portions 27 are disposed on tops of the convex portions 28 to bond the convex portions 28 with a back face of the outer panel 22 through the intermediary of the bonding portions 27. Still more, a lock reinforcement 30 and hinge reinforcements 31 are joined to the concave portion 25 as reinforcing members of the automobile hood 21B.
As shown in FIGS. 17(a) through 17(c), an automobile hood (wavy bead-type hood structure) 21C is constructed by an outer panel 22 having a predetermined curvature and an inner panel 23 having a concave portion 25 at a peripheral part thereof and a plurality of wavy beads 29 disposed in parallel in a longitudinal direction of the vehicle at the center part of the automobile hood 21C. The automobile hood 21C has a cross-sectional structure in which the outer panel 22 is joined with the inner panel 23 while interposing airspace portions 24 by joining the peripheral part of the outer panel 22 with an end of the concave portion 25 of the inner panel 23 by means of a hemming process. Further, bonding portions 27 are disposed on tops of the wavy beads 29 at predetermined intervals to bond the wavy beads 29 with a back face of the outer panel 22 through the intermediary of the bonding portions 27. A lock reinforcement 30 and hinge reinforcements 31 are also joined to the concave portion 25 as reinforcing members of the automobile hood 21C.
The pedestrian protection performance of the automobile hood is evaluated by a head injury value (abbreviated as an HIC value hereinafter)) in general. The HIC value is given by the following equation (1) (a maximum value of a product of 2.5 power of average acceleration within an arbitrary time and occurrence time). The smaller the value of HIC, the better the pedestrian protection performance is.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                                                                                                            ⁢                                                H                  ⁢                                                                          ⁢                  I                  ⁢                                                                          ⁢                  C                                =                                                                            (                                                                        t                          ⁢                                                                                                          ⁢                          2                                                -                                                  t                          ⁢                                                                                                          ⁢                          1                                                                    )                                        [                                                                  1                        /                                                  (                                                                                    t                              ⁢                                                                                                                          ⁢                              2                                                        -                                                          t                              ⁢                                                                                                                          ⁢                              1                                                                                )                                                                    ⁢                                                                        ∫                                                      t                            ⁢                                                                                                                  ⁢                            1                                                                                t                            ⁢                                                                                                                  ⁢                            2                                                                          ⁢                                                  a                          ⁢                                                                                                          ⁢                                                      ⅆ                            t                                                                                                                ]                                    max                  2.5                                                                                                                                                  (        1        )            
Where, “a” is a triaxial combined acceleration (unit is G) at a center of gravity of a head, t1 and t2 are times when the value of HIC is maximized at time t when 0<t1<t2 and a calculated time (t2−t1) is determined to be 15 msec., or less.
FIG. 14(a) is a diagrammatic view explaining a state when a head collides against the prior art automobile hood (the beam-type hood structure 21A, see FIGS. 13(a) through 13(c)), FIG. 14(b) is a graph explaining a relationship between acceleration a and time t when the head collides against the automobile hood and FIG. 14(c) is a graph explaining a relationship between the acceleration a and a stroke (a displacement of the head thrust into an engine room when the head collides against the automobile hood) S. The acceleration generated when the head of the pedestrian collides against the automobile hood may be roughly categorized into primary impact acceleration caused when the head collides against the automobile hood and secondary impact acceleration caused when the automobile hood 21A butts against built-in parts within the engine room. Noted that even though a relationship of degrees between the primary impact acceleration and secondary impact acceleration varies more or less also in the automobile hood (cone-type hood structure) 21B in FIGS. 16(a) through 16(c) and the automobile hood (wavy bead-type hood structure) 21C in FIGS. 17(a) through 17(c), they present the relationship between the acceleration a and the time t and the relationship between the acceleration a and the stroke S similar to those shown in FIGS. 14(b) and 14(c).
Meanwhile, the automobile hood must meet requirements of basic performances required since the past such as a tensile rigidity, a dent resistance, a bending stiffness, a torsional stiffness and the like. The tensile rigidity is necessary to suppress elastic deformation of the automobile hood that is otherwise caused when wax is applied to the hood or when the hood is pressed down to lock. The tensile rigidity is determined by Young's modulus and thickness of the outer panel as well as by position where the outer panel is joined with the inner panel (bonding positions of the bonding portions 27 in FIGS. 13, 16 and 17). The dent resistance is necessary to suppress plastic deformation of the hood that is otherwise caused by and remains due to a fly rock and the like and is affected by proof strength and the thickness of the outer panel. The bending stiffness is necessary to suppress elastic deformation of the peripheral part of the automobile hood that is otherwise caused by lead-in force in locking the automobile hood and by reaction force of a cushion rubber, a damper stay, a sealing rubber and the like and is affected by shapes (secondary moment in area) of the inner panel and reinforcements at the peripheral parts of the automobile hood and the Young's modulus. The torsional stiffness is affected by the bending stiffness of the peripheral part of the automobile hood and the thickness and the shape of the inner panel at the center part.
Although the automobile hood is required to meet both of those basic performances and the pedestrian protection performance, it is often difficult to meet the pedestrian protection performance by a hood whose material, thickness and shape are designed so as to meet only the basic performances because the space under the automobile hood, i.e., the space between the automobile hood and structures such as the engine, is limited.
Then, when the space under the automobile hood is small in the cases of the prior art automobile hoods 21A, 21B and 21C described above, the acceleration generated at the secondary impact is greater and its duration is longer than the acceleration generated at the primary impact as shown in FIGS. 14(b) and 14(c), so that the acceleration of the secondary impact negatively affects the HIC value calculated by the Equation (1) described above (the HIC value does not reach a satisfactory level). Still more, the concave portion 25 is hardly crushed and deformed because the peripheral part of the automobile hoods 21A, 21B and 21C where the bending stiffness is required in particular must have the increased second moment of area of the concave portion 25 of the inner panel 23 and the reinforcements 30 and 31 and must assure the tensile rigidity. Therefore, there has been a problem that it is difficult to reduce the acceleration caused at the time of the secondary impact and to improve the pedestrian protection performance.
One measure for solving this problem by a hood structure of the automobile hood is to decrease the acceleration caused by the secondary impact by fully assuring an energy absorption amount at the time of the primary impact. Although it is conceivable to increase thickness of the panel as a method for realizing this measure, there have been problems that cracks may occur at a hem portion 22A in processing the hem and R of the hem portion 22A increases, thus harming its look, as shown in FIG. 15(a) when the thickness of the outer panel 22 is increased.
When thickness of the peripheral part (the concave portion 25) of the inner panel 23 is increased as shown in FIG. 15(b) on the other hand, the secondary impact G with the built-in parts, i.e., the structures such as the engine, increases and the HIC value aggravates as a result in contrary because a crush-deformation load increases. Specifically, because the concave portion 25 of the peripheral part is required to have a high bending stiffness, a sectional height “h” of the concave portion 25 cannot be reduced even when the thickness thereof is increased. Therefore, there has been a problem that although an energy absorption amount at the time of the primary impact increases, a rise in the acceleration at the time of the secondary impact becomes sharp and the stroke decreases as shown in FIGS. 15(c) and 15(d), thus leading to the aggravation of the HIC value. It is noted that in FIGS. 15(c) and 15(d), broken lines represent the case when the automobile hood using the inner panel in which the thickness of the peripheral part (the concave portion 25) is not increased is used.
Another measure for improving the pedestrian protection performance is to hold or to increase the energy absorption amount at the time of the primary impact and at the same time, to lower the crush-deformation load of the hood to reduce the acceleration at the time of the secondary impact. In order to realize this measure, it is conceivable to form the peripheral part (the concave portion 25) of the inner panel 23 into a readily crushable shape or more specifically, to relax the shape of the concave portion 25 of the inner panel 23 (to moderate inclination angles θ1 and θ2 of the side walls of the concave portion 25). However, this method also has had a problem that the tensile rigidity of the outer panel 22 becomes insufficient because a distance L for joining (bonding) the outer panel 22 with the inner panel 23 extends and an elastic deformation volume D of the outer panel 22 increases because a cross-section of the concave portion 25 cannot be reduced to ensure the bending stiffness of the automobile hood.
In view of the problems described above, the present invention seeks to provide an automobile hood that meets the basic performances required to the automobile hood and to the peripheral part of the automobile hood in particular and that excels in the pedestrian protection performance.